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Old 22nd November 2004, 06:46 AM   #1
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Question 50C5 add-on amp

Hello All, 1st post....er, ah, question.
I want to add a 50c5 tube to my already built one tube (6BM8) AM transmitter to monitor the broadcast. I was thinking about an LM386 but I have one extra 7 pin socket and lots of 50c5 tubes. It has 160VDC and 250VDC available. I want to use the least amount of parts as space is limited. It will have a pot and small speaker and this circuit will not be used as part of the transmitter, as said, only to monitor the outgoing music. Any help with a schematic? I've seen several here but not quite as simple as I want. Thanks.
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Old 22nd November 2004, 06:15 PM   #2
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I suspect the big problem will be: you need 50V and 0.150 Amps to heat-up a 50C5, and I doubt you have that available.

The basic plan for any table-radio output is here: Plan

You should increase the 0.002 to 0.01 or better if you want bass, but 0.002/0.47K is reasonable for a 4" speaker and small output transformer. Using a large cap with a small transformer will just give you fuzzy tubby bass.

The transformer impedance and ratio isn't very critical, unless you need optimized results. 1K or 6K will clip a little early, something like 2K is best but 1.6K or 3K will be just as good. Your real choices are: generic tube-radio output transformer for $5-$25, or hi-fi iron at $100+++.

You can bypass the 150 ohms with about 50uFd 50V for more gain, and you will probably have to: a 50C5 without a voltage amp needs many volts of drive, around 5V RMS for full output even with cathode capacitor.

You could omit the 30uFd and move the 1200 ohms to the other side of the primary. That gives triode mode, with some damping and lower distortion, but lower gain so you need more signal.

A 50C5 runs HOT. There is 7.5 watts in the heater and generally over 5 watts in the plate. With other losses, this means nearly 15 watts in a very small space. Historically you put such output tubes FAR away from tuned circuits so the heat does not drift the tuning. If your transmitter is smaller than a table radio, I think a 50C5 will give you heat-trouble.
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Old 22nd November 2004, 06:28 PM   #3
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Quote:
Originally posted by PRR
generic tube-radio output transformer for $5-$25, or hi-fi iron at $100+++.
If you don't already have some kicking around i do...

Quote:
you will probably have to: a 50C5 without a voltage amp needs many volts of drive, around 5V RMS for full output even with cathode capacitor.
The 50EH5 needs less drive (i'm using a pr in a spud amp and have no problems driving it to clipping).

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Old 22nd November 2004, 07:23 PM   #4
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mmm, that LM386 is sounding better. Thanks for the response. I am concerned about heat and DRIFT , as this is a small bakelite cabinet. Any other 7 pin suggestions. Sound quality is not important only need to know there is output on the transmitter. Thanks
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Old 24th November 2004, 05:09 AM   #5
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> that LM386 is sounding better

Don't say that here!!! These guys will make you go stand in the Chip-Amps corner. And those guys will laugh at your puny LM386. (They never heard my copper-sinked 20% over-volted LM379 amp...)

>> generic tube-radio output transformer for $5-$25

> If you don't already have some kicking around i do...

I kicked that stuff out of my life some years back. Fun, but....

If you want to give/sell Mike some iron, cool; otherwise it is $5-$25 for a basic kitchen-radio output depending how hard he shops and how bad the postage is. (Actually the deal would be a 1960s kitchen-radio for $0.50/OBO. But they are getting harder to stumble into.)

> The 50EH5 needs less drive

Several "better" tubes. 12FX5/60FX5 is killer SEP. But if Mike has 50C5, it should "work", more or less. And if it works but he wants more, then he can spend the big bucks ($5?) for a 50EH5 or other super-phono output. In this case, I think he wants less.

> Any other 7 pin suggestions. Sound quality is not important only need to know there is output

Sure. Use the kitchen-radio transformer with any RF/IF pentode, like 6AU6 or 6BA6, the standard IF from a 5-tube radio. With 6BA6, try 100V supply, 180 ohm cathode resistor, about 8ma plate current and 3mA G2 current. Optimum load is around 10K, but 2K5:4 ohm transformers are a lot easier to find. Load with an 8 ohm speaker, you get a 5K primary with around 500Hz cut-off (assuming a typical 200-250Hz cutoff at rated impedances). Maximum power output will be like 150mW at like 30%THD (6BA6 is very nonlinear, 6AU6 may sound nicer) with just over 1V RMS drive. But if you just want to know "it's live!", less than 50mW may be plenty and THD may be under 10% and drive is well under 1V. If you also size the grid-cap to cut everything below 500Hz, you get about 5dB more intelligibility for the same watts, and also reduce the effect of the gross THD number. Throw 0.01uFd across the primary to take some of the screech out, and it will whisper program at about 3 Watts total heat.
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Old 24th November 2004, 07:10 AM   #6
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I made a SE headphone amp with a pair of 60FX5's,with the circuit from the RCA manual..After a bit of tweaking it sounded darn good,and had plenty of power to make your ears bleed (1W for headphones is overkill-and-a-half!)
The problem I had was that *all* the 60FX5's I found were really microphonic..For instance,if I have it on my table here,I can hear myself typing!! Maybe I just happened into a bad bunch?
The 12FX5 might be worth a shot,if you have 12V available and sufficient current.
Edit: I should also mention that the 12/60FX5 has lots of gain.
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Old 24th November 2004, 04:06 PM   #7
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Thanks guys. I have all the junk I need, with an AA5 Bakelite habit I picked up from ebay. I'll try some of your circuits on another project and see how it goes. I have an open transmitter I can try the 50C5 on that heat won't hurt.
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Old 25th November 2004, 08:01 AM   #8
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> SE headphone amp with a pair of 60FX5's

My first stereo (I grew up with mono) was a hot-chassis rig with two 60FX5s (no preamp tube needed with a hot crystal needle and 60FX5) and cardboard speakers with Fostex drivers (long before Fostex became faddish).

Dang sweet rig, considering.

> *all* the 60FX5's I found were really microphonic..

> mention that the 12/60FX5 has lots of gain.

Yes and yes, and they go together. 12/60FX5, like 8417 and (to a lesser degree) 7591, have the grid "too close" to the cathode. This increases gain (and allowed tubes to compete with transistors a little longer) but makes them sensitive to vibration. If the makers really cared, they would have used more grid-support, but that can cut gain and anyway: 60FX5 was for cheap rigs, who cares? (8417 can be microphonic, more than say 6L6 or 6550, but being a "serious" tube they did have some grid support and microphony was not a big problem.)

> Maybe I just happened into a bad bunch?

I suspect, with zero evidence, that all 60FX5s were built on the same line with the same tooling, no matter what name was on the box. No maker could tool-up for all tube types, but liked to have a fairly complete list to be able to fill most requests. Unless a tube was needed in large numbers, second-sources just bought it from the company who developed it, and put their brand on. 60FX5 must have sold thousands but not millions, not enough that the world needed multiple factories making it. And it was only made for a few years: before transistors, tube makers liked to sell a 12AX7-2x50C5 lineup for hot-chassis stereo phonograph, after transistors got cheap nobody used tubes at the 1-Watt level (my second 1W stereo was 4 transistors per channel, bootstrapped out the wazzoo). But there were a couple of years when a super-gain minimalist tube amp was marginally cheaper than trannies.

If you can't type softer, mount the 60FX5 the same way you mount a preamp tube, on rubber bushings, maybe with a heat-dissipator both for mechanical damping and so the 60FX5's heat does not cook the bushings so fast. Or set the amp on a pillow, or a shelf away from the typing table. Or just enjoy the funkiness of tubes; they have soul, and life, unlike dull transistors. (But maybe cat-whisker radio fans said the same thing when reliable tubes started replacing touchy cat-whisker radios....)
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